1. Field of the Invention
This invention relates to the restoration of aqueous solutions of polymers used as quenching baths. Quenching baths are used in the heat treatment and tempering of steels and other ferrous alloys.
2. Prior Art
One of the methods for heating treating steels and ferrous alloys consists of heating the metal to a relatively high temperature and then quenching or cooling the metal at a controlled rate. The rate of cooling is controlled by immersion in a liquid of controlled composition. Quenching oils and salt quenching baths are well known in the art. More recent on the scene have been quenchants which are aqueous solutions for example of polyvinyl alcohol, polyglycols or polyvinyl pyrrolidone.
With the polymeric quenchants one of the methods for controlling the quenching severity or rate of cooling is by varying the concentration of the polymer in the solution. That is, pure water conducts heat away from the hot metal being quenched very rapidly. As the concentration of the polymer increases the rate of heat removal by both conduction and convection slows. This is primarily the result of viscosity increase about the metal part and the increase in polymer deposition on the part being quenched as the concentration increases.
The polymer is added to the water in the quench bath to decrease the rate at which the heat is removed from the metal articles immersed in the bath. An increase in the concentration of the polymer causes a decrease in the rate of heat removal by convection and a decrease in the heat removal by conductivity in the solution.
This simple relationship, however, gets distorted during the use of the quenching bath. The cooled metal parts tend to preferentially drag out the higher molecular weight polymer molecules. Also as the heated metal enters the bath there is a certain amount of cracking of the higher molecular weight polymer molecules into fragments with lower molecular weights.
The preferential removal or cracking of larger molecular weight polymer molecules leads to skewing of the normal molecular weight distribution. The skew in the molecular weight distribution of the polymer can be observed using gel permeation chromatography. At a given concentration of polymer this skewing towards lower molecular weights results in a lower viscosity and a higher conductivity in the quench bath.
As a result of these effects accurate control of quench severity becomes difficult when a bath has been used for any significant period of time. The quench severity of a bath cannot be controlled by simply measuring the concentration of the polymer by a method such as refractive index. The rate of heat removal from the metal articles is a function of more than just concentration. Measurements of viscosity of the bath while more accurate than concentration measurements still do not give any clue as to the rate at which heat is conducted away from metal articles in the bath.